We will investigate the mechanisms by which exogenous microbial, liposomal or particulate antigens are processed for presentation by class I MHC (MHC-I) molecules to elicit CD8 T cell responses. Soluble exogenous antigens do not efficiently elicit CD8 T cell responses, but our previous studies demonstrated that particulate antigens, such as liposomes and bacteria, were presented by MHC-I molecules to elicit CD8 T cell responses. Our hypothesis, supported by preliminary data, is that this processing occurs via a mechanism that is distinct from the classical MHC- I processing pathway. To test this hypothesis, we are exploring the subcellular mechanisms of this alternative MHC-I processing pathway for exogenous antigens. We aim to define the intracellular compartments involved in processing and the subcellular site where peptide-MHC-I complexes are formed. In addition, we are optimizing the formulation of liposomes to elicit CD8 T cell responses, and we are testing new non- viable particulate antigen preparations that may be useful for immunization to achieve CD8 T cell responses. CD8 T cell responses are crucial to the immune response against many infections and tumors, and they may be elicited by viable viral vaccines. However, there are many diseases where safe viable vaccines are not available, and immunocompromised patients (e.g. with immunosuppression due to tumors, chemotherapy, or AIDS) can also succumb to severe infection even with attenuated strains of microbes used for vaccination. Thus, the ability to elicit CD8 T cell responses using non-viable vaccine preparations is an important advance. In addition, our studies indicate that extracellular or intravacuolar bacteria, such as Salmonella (a vaccine vector candidate), may be used in some cases to elicit MHC-I restricted T cell responses. These studies will be of significance in the development of vaccines for infectious diseases as well as immuno-therapy for tumors.